Recently it is needed more and more to measure the purity of water and very weak change in a property of water precisely without changing the quality of water. For example, in a fabrication process of a semiconductor, the water purity is required to have a level of specific resistivity near the theoretical limit. Further, so-called functional water, wherein a specified function is added to ultrapure water having very high purity, is used recently.
Spectroscopic analysis is used very effectively for qualitative and quantitative analysis of water and aqueous solutions in various ways. The spectroscopy is classified into ultraviolet and visible spectroscopy, near infrared spectroscopy and infrared spectroscopy according to measurement wavelength region.
In near infrared spectroscopy, absorption spectra of water due to hydrogen bonds inherent to water are observed remarkably in 800 to 1400 nm. It is proposed in, for example, Japanese patent laid open Publication H3-175341 to use the spectra for measuring solute components in water. In the liquid state water molecules are bonded to each other with hydrogen bonds between them, and the state of hydrogen bonds are changed sensibly when solute components are mixed into water. By studying the change, the mixed components can be analyzed quantitatively. In concrete, when an inorganic electrolyte is dissociated as ions in water, bonding states of water molecules themselves and those between water molecules are affected due to, for example, cutting or distortion of hydrogen bonds between water molecules around an ion and a bulk water molecule caused by hydration of ions, polarization of water molecules due to electric fields of ions, and the like. Then, the near infrared spectra of the aqueous solution become different from those of pure water. By calibrating the change in spectra beforehand, the concentrations of the ion species can be determined quantitatively based not on the absorption spectra due to the ion species, but on the change in absorption spectra of water.
Recently, it is proposed to measure the concentration of hydrated substances in an aqueous solution quantitatively with far ultraviolet spectra of water which is closely related to the bonding state of water as in near infrared spectra (Japanese Patent laid open Publication 2005-214863 and APPLIED SPECTROSCOPY Vol. 58, No. 8 (2004) 910-916). In order to discriminate an aqueous solution and to measure very small concentrations of solutes quantitatively, the method uses that absorption spectra due to n→σ* transition of water having a peak around 150 nm shifts toward the longer wavelength side due to the electric field formed between water itself and hydrated ions in the solution, and that a part of the spectra appears in a region which can be measured with a conventional spectroscopic apparatus (or a spectroscopic apparatus which does not need evacuation). The absorption spectra of water in the far ultraviolet region has a much higher sensitivity for detection and for quantitative analysis on very small solute concentrations, but it is used only in an a wavelength range longer than 180 nm or a limit of absorption spectra measurement because the absorbance of water itself is very large.
Attenuated total reflectance (ATR) is explained below because it is used in the invention in order to measure absorption spectra of a material having very large absorption. According to an attenuated total reflectance method, penetration of light of the order of wavelength (evanescent wave) on total reflectance at the surface of an optical probe is measured, and absorption spectra in correspondence to a cell length of the order of wavelength can be obtained. In Japanese Patent laid open Publication S62-75230 a method is proposed for measuring solutions with high solute concentrations with an optical probe for attenuated total reflection spectra. Various attenuated total reflectance methods have been used by using an optical probe made of synthetic quartz, sapphire or the like. In Japanese Patent laid open Publication H7-12716 it is proposed to enhance measurement sensitivity of the attenuated total reflection method itself.
An optical probe for attenuated total reflectance made of a plurality of optical materials is also proposed. In an infrared optical system for optical analysis proposed in the U.S. Pat. No. 5,703,366, an optical probe reflects a light totally in a plane in contact with a sample substance. In order to overcome disadvantages of a probe made of one crystal member, such as chemical resistance, mechanical properties or high cost, a probe has a first crystal member and a second crystal member in contact with the first one. The second crystal member has a plane in contact with a sample substance. The two crystal members have substantially the same refractive index. For example, if the second crystal member is diamond which transmits infrared light, the first crystal member is, for example, zinc selenide (ZnSe).
In an optical element for transmitting infrared light described in Japanese Patent laid open Publication S64-56401/1989, a diamond thin film or a diamond-like carbon (DLC) thin film including a diamond structure of, for example, 600 nm thickness is formed on a surface of an optical element made of a material such as SiO2 or ZnSe which transmits infrared light, in order to improve the surface strength and humidity resistance. One example of the optical element is a multi-reflection prism provided in an attachment for an apparatus for attenuated total reflection measurement. As to the optical properties of a DLC thin film used in the embodiment, it is only stated that the infrared absorption spectra are not affected, except results on abrasion test and humidity resistance. That is, only mechanical and chemical properties attract attention on the function of a diamond thin film.
Further, in a method described in Japanese Patent laid open Publication H5-332920/1993, an analysis surface of a sample (silicon wafer) is arranged to contact with air, while the opposite surface thereof is closely attached to a prism made of a soft solid material having refractive index smaller than that of the sample. Then, an infrared ray of light is incident on the solid material to be reflected at the analysis surface, and attenuated total reflection spectra are measured at the sample surface having refractive index larger than the prism material. In this method, it is supposed that the second layer (sample) in the composite materials transmits infrared light.
It is proposed on an attenuated total reflection optical probe, in Japanese Patent laid open Publication 2001-91710, that a second layer having large optical absorption, such as zinc oxide or tin dioxide, has mirror adhesion to a transparent first layer such as silicon. The second layer contacts with a sample. An optical material having a larger refractive index is used for the first layer, and another optical material having a smaller refractive index is used for the second layer. However, if the end face angle and incident angle described in the first embodiment and the like are used, the ray of light incident on the first layer is reflected totally at the interface between the first and second layers, or only a part of the evanescent waves can penetrate into the sample located at a side of the second layer opposite to the interface. Further, because the optical absorption of the second layer is large, absorption can be measured with a very low signal-to-noise ratio eventually. The concept of this probe is not clear.
As explained above on the prior art prisms made of a plurality of optical materials, it has been considered that an attenuated total reflection prism can be fabricated only if the refractive index of a second layer in contact with a sample is equal to or larger than that of a first layer. In any probe, it is supposed that the refractive index of a sample in contact with the interface of the composite prism is smaller than that of the first layer in the prism. The optical materials are selected under a condition that they transmit light such as infrared light. Japanese Patent laid open Publication 2001-91710 describes an example of a composite prism wherein the refractive index of the second layer is smaller than that of the first layer. In this case, the attenuated total reflection occurs at the interface between the first and second layers, and total reflection spectra at the sample surface cannot be measured. The concept of this probe is not clear.
Absorption of water in the near infrared region is weak because it is due to an inherently forbidden transition, and very small solute concentrations in water cannot be measured. Therefore, a method is needed for measuring such very small solute concentrations which cannot be measured in near infrared spectra. On the other hand, water has a large absorption peak near 150 nm, and solutes can be detected and their concentrations can be measured due to a change in the absorption spectra at a much higher sensitivity than that of near infrared spectra. However, when spectra of water or an aqueous solution is measured in far ultraviolet region, the absorption of water is a large obstacle for spectroscopy. As to other substance than water having large absorption in the far ultraviolet region, the absorption becomes a large obstacle for spectroscopic measurement similarly. The above-mentioned attenuated total reflection measurements for the infrared and visible regions cannot be used in the far ultraviolet region because transmittance is not sufficient high or because the optical probe does not cause total reflection at the flat plane in contact with a sample substance.